Transponder systems are applied in an increasing number of fields; specifically sensor transponders are of interest here. The combination of RFID technology and sensor technology opens up new possibilities in many technological fields, for example in medical technology. In medical technology applications, sensor transponder technology opens up valuable possibilities of treating diseases of the cardiovascular system in humans, such as cardiac insufficiency. However, miniaturized sensor transponder systems passively supplied with power may also be employed, and are of great interest, in other technical fields, e.g. in the fields of construction physics, chemical industry and automobile industry. In construction physics, their utilization is feasible for monitoring the level of humidity in wood constructions or the insulating properties of so-called vacuum insulation panels. In the chemical industry, pH values in liquids may be monitored. In electric vehicles, such miniaturized transponders introduced into batteries might monitor temperatures and pressures and thereby increase safety.
However, it has been found that the RFID transmission technique is not well suited for sensor transponders since the higher current requirement of such a transponder, the useful broad range of transmission at small dimensions, and possibly existing lossy materials that might surround the transponder set limits to the RFID technique. Even though several sensor transponders have already been known, their range of transmission is small, so that their application possibilities are limited.
FIG. 1 shows a schematic representation of a known sensor transponder system comprising a passive transponder, implanted into a patient's body, for providing measurement signals. By reference to such a system, the disadvantages of conventional passive transponders shall be discussed below, said disadvantages occurring, in particular, in environments where the transponder is surrounded by lossy materials, such as human tissue, for example. In the context of such applications, medical studies have shown that the treatment of cardiovascular diseases may be clearly improved by constant monitoring of parameters, such as blood pressure, temperature, etc. Constant monitoring of cardiovascular functions supports the optimum adjustment of drug dosage for a patient. Sensor transponders implanted into a human body may therefore support and, thus, considerably improve a therapy. Said transponders may be located at different positions inside the body and may monitor the performance of the cardiovascular system. So-called passive transponder systems are of particular interest in this context, since the above-mentioned transponders typically remain inside a patient's body for a relatively long period of time, and therefore, conventional power supply by means of a local battery is not possible. More specifically, for active transponder systems it would be useful to provide large batteries that enable energy supply over a sufficiently long period of time. Due to the available transponder dimensions, this is typically not possible at all. Alternatively, the implanted transponders might be replaced or removed after a predetermined time period and be fitted with new batteries, which is undesirable, however, since this would involve unnecessary interventions for the patients. Thus, utilization of transponders which have a local battery is not desirable in this field. In medical technology applications it is desirable, in particular, to design the dimensions of the transponder such that so-called catheter implantation is possible, i.e. a minimally invasive approach to inserting the transponder into the human body. Consequently, antennas may be used which have the shape of a rod and have a size of only few millimeters. Such transponders, which comprise the useful sensor technology for detecting the desired parameters, consume considerably more energy than conventional ID transponders as are employed in conventional RFID systems.
Unlike conventional RFID systems, the maximally possible distance between the reader and the implanted transponder is also to be considered, so that such systems are also suitable for corpulent patients. Data transmission may be realized over the useful distance and at the useful data rate. For example, to obtain a meaningful medical diagnosis, it may be ensured that a pressure development may be transmitted for a duration of several heartbeats.
As was already mentioned above, FIG. 1 shows a conventional transponder system as is already employed today in medical technology applications. The system comprises a passive transponder 100 implanted into the body of a patient 102, as is schematically shown in FIG. 1. The transponder 100 comprises an antenna 104 having a corresponding antenna inductance LT. A capacitor CT is connected in parallel with the antenna 104. A series connection consisting of the resistor R and the switching element 106 is further arranged in parallel with the antenna 104 and with the capacitor CT. The switching element 106 is a transistor, for example, that connects or disconnects the resistor R in parallel with the antenna 104, depending on a control signal that is present and is provided via a line 108. In addition, the transponder 100 comprises the chip 110 depicted in FIG. 1, whose inputs are connected to the antenna 104 via corresponding connections 112 and 114. Said connections have, firstly, the capacitor CT and, secondly, the series connection consisting of the resistor R and the switching element 106 arranged therebetween. The chip 110 comprises the sensor technology, not depicted in detail in FIG. 1, for detecting the desired parameters of the patient 102, and the useful processing electronics for processing said parameters for transmission. The chip 110 further comprises a controller for providing, via the line 108, corresponding control signals for controlling the switching element 106 for a load modulation, which will be explained in more detail below.
The transponder system further comprises a reader 120, which also comprises an antenna 122. The antenna 122 of the reader 120 is fed by a voltage supply 124 connected to the antenna 122 via the lines 126 and 128. A resistor R is arranged within the line 126. In addition, lines 130 and 132, which are connected to corresponding inputs of a receiver 134, branch off from the line 126. The line 132 comprises a capacitor C connected between the lines 126 and 128 and, thus, in parallel with the antenna 122.
The region 136 depicted in dashed lines in FIG. 1 (also referred to as a transmission channel) schematically represents that region within which energy is transmitted from the reader 120 to the transponder 100. More specifically, the reader 120, which is located outside the body of the patient 102, generates an alternating magnetic field BR which penetrates the human body 102. Part of the magnetic flux establishes a coupling to the antenna coil 104 of the sensor transponder 100. As a result, a voltage which supplies the transponder chip 110 with energy is induced within the transponder 100. The parallel resonant circuit consisting of the antenna inductance LT, the capacitor CT and the resistor R enables exploitation of the effect of the increase in resonance and results in efficiency improvement. The reactive current IT leads to a magnetic field BT with opposite orientation, so that the voltage across the reader coil, or reader antenna, 122 is attenuated.
The transponder system described by means of FIG. 1 uses so-called load modulation in order to transmit data back to the reader 120. For this purpose, the resistor R is provided, which may be optionally connected as a load, via the switching element 106, to the resonant circuit consisting of the antenna inductance LT and the capacitor CT. In this manner, the reactive current IT and the magnetic field strength at the transponder 100 may be changed. This change is detected at the antenna 122 of the reader 120 by the receiver 134. In practice, this change is relatively small as compared to the field BR of the reader 120 and as compared to the noise.
In transponder systems, one differentiates between two ranges; firstly, the so-called energy range, and secondly, the so-called data range. The energy range characterizes the maximum distance at which the transponder 100 may be provided with sufficient energy to function properly. The reading range characterizes the maximum distance at which the reader 120 may receive data signals from the transponder 100. With simple ID transponder applications, the reading range is typically much smaller than the energy range, but in the example depicted in FIG. 1 both distances may be at least up to 40 cm.
The discussion which follows shows that conventional transponder systems as are shown in FIG. 1 are unsuitable. The dimensions of a transponder antenna implantable by means of catheters are limited to 2 mm×8 mm. During transmission of the energy through the human tissue, there will naturally be losses, and both the energy losses and the limited dimension of the antenna will result in that the so-called mutual inductance between the antennas 104 and 122 is reduced. The mutual inductance describes the magnetic coupling between the reader 120 and the transponder antenna 104, so that reduction of the mutual inductance consequently also implies a reduction of the maximally possible distance between the transponder 100 and the reader 102. As was mentioned, the desired distance in medical technology applications is advantageously 40 cm. Transmission of the energy from the reader 120 to the transponder 100 is useful over said distance in order to enable the transponder to function.
Such an energy range may be achieved in that the antenna structures are optimized and in that, in addition, a correspondingly high level of transmission power is employed. In one example, the transponder antenna 104 may comprise a 1.5 mm×8 mm ferrite rod which has 20 windings of a wire. The antenna 112 within the reader 120 may have a winding of a diameter of 26 cm and consist of a 2.5 cm copper tubing or a 3 cm copper tape, for example. At a frequency of 6.78 MHz it is possible to achieve good results in terms of energy transmission. Good results are also achievable at 13.56 MHz. The frequencies mentioned represent a fair compromise between loss effects inside a human body and the antennas; however, said loss effects also increase as frequencies increase. In addition, for higher frequencies, higher voltages are induced within the transponder 100, it being useful, for example, for ensuring sufficient supply of a transponder chip 110, to provide a voltage amplitude of 240 V across the reader antenna 112.
The sensor transponder 100 is to measure several physical parameters, e.g. blood pressure, temperature, and the supply voltage within the transponder. To enable medical diagnosis, it is desirable to transmit the pressure development of heartbeats at a sufficient resolution. Typically, a pressure ranging from 750 hPa to 1150 hPa may be assumed, the resolution intended to be ±1.3332 hPa. In addition, negative effects of subsequent signal processing are to be taken into account, such as quantization of the signal received within the receiver 134 by an analog-to-digital converter and, in this context, in particular the noise, so that a resolution of, e.g., 10 bits, which corresponds to about 1024 stages, is to be used. To reconstruct the pulse shape of the heart pressure in the reader 120, a time resolution of 100 samples per second is useful, which corresponds to a sampling interval of 10 ms. In addition, a temperature value is to be detected, e.g. with a resolution of 8 bits. The internal temperature of the human body is more or less constant, so that, here, a sampling rate of 1 per second is sufficient. Also, a voltage of the power supply 124 may be known, since the sensors within the transponder 100 are dependent on the voltage. Thus, measurement data with regard to the temperature at a resolution of 8 bits may be taken 10 times per second, so that, together with the data rate for the pressure values, an overall data rate adds up to about 2.09 kbit/s. Moreover, the transmission protocol exhibits a certain complexity, which may also be taken into account. Due to power restrictions, parallel measurement of data transmission may not be possible in some sensor transponders as are known in conventional technology, so that higher data rates are useful in such a case. For the following deliberation, a data rate of 13 kbit/s shall be assumed.
The above-mentioned reading range of the transponder is limited by several factors. The magnetic coupling between the antennas 104 and 112 may be small, which results in a small signal strength at the reader antenna 112. In addition, the sensitivity of the receiver 134 restricts the reading range. Also, the voltage amplitude caused by the generator 124 at the reader antenna 112 is relatively high as compared to the transponder signal, which also makes signal processing more difficult. Furthermore, the noise of the power amplifier within the receiver 134 is more intense than the transponder signal, so that in the event that the signal/noise ratio (SNR) may fall below a minimum, decoding of the transponder signal will no longer be possible. In addition, in such a field of application, the noise created by the person or patient as well as antenna movements caused by a movement of the patient will also interfere with data transmission.
What follows is an explanation of the transfer function for the transponder system shown in FIG. 1, said transfer function being useful to analyze the transmission channel so as to find out, e.g., a signal strength characteristic to be expected as well as further channel characteristics. To derive the transfer function, the equivalent circuit diagram, shown in FIG. 2, of the transponder system shown in FIG. 1 shall be used. The resonator circuit of the transponder 100 is modeled by the voltage source VT, which may be varied by the modulation resistor R. The transmission channel 136 is depicted by an equivalent circuit of a transformer, wherein the resistors RR and RT represent transmission losses. The antennas 104 and 112 are represented by the inductances LT-M and LR-M, the inductance M modeling the mutual inductance between said antennas. The generator 124 is depicted by its internal resistor RG and represents the voltage VC.
The transfer function VC/VT may be derived by solving Kirchhoffs mesh law, and the result is a first-order bandpass function. When assuming LR=409 nH, RR=9.8 mΩ, C=1.1 nF, and M=0.114 nH, a transfer function as is shown in FIG. 3 will result with a distance of 30 cm.
Switching of the load resistor R results in an amplitude shift keying modulation, so that upper and lower sidebands will occur within the frequency range. The generator signal transmitted from the reader 120 to the transponder 100 acts as a carrier for data transmission in the opposite direction. At 26 kHz a Manchester-coded 13 kbit/s signal has components in the baseband. At the corresponding sideband frequencies, the transmission ratio is about 0.000196. In the event of a modulation voltage of 1 V on the transponder side, about 200 μV are achieved at the reader antenna, effective voltage being limited to about 114 μV.
In a transponder system as is shown by FIG. 1, various noise sources exist as well, the transponder signal being superimposed by noise voltages and noise currents. Such noise sources are located, e.g., in the frequency generator, in the power amplifier, in the antenna, and in the receiver.
The power amplifier also adds a noise caused by the shot noise of the pn junctions and by the so-called Johnson noise of the resistors. This results in that an effective voltage at the receiver is of interest which, with a conventional power amplifier having a noise figure of 16 dB, amounts to about 2.3 mV. The gain of the parallel resonant antenna circuit causes amplification of the noise near the resonant frequency. FIG. 4(a) shows an equivalent noise circuit diagram consisting of a signal source with an amplifier VS and the noise source ES, the internal resistor RS, the antenna resonant circuit LP and CP, and the receiver input noise sources EN and IN. The spectral noise density VSO is composed of the voltage noise density and the current noise density. The noise voltage may be estimated by a simulation, which also yields the system gain as well as the input and noise voltage densities within the frequency range of interest. The resulting noise voltage while using the equivalent noise circuit diagram of FIG. 4(a) is shown by means of FIG. 4(b), when a system having the above-described parameters is taken as the basis.
The mean-square noise voltage may be determined by using a simulation, e.g. a Spice simulation. In addition, the simulation provides the system gain as well as the input and output noise voltage densities within the frequency range of interest. From FIG. 4, a spectral shaping of the white noise is to be detected as input noise, the noise density being at its maximum near the generator frequency of 6.78 MHz. The effective noise voltage is obtained by integrating the noise density over the receiver frequency range. When taking as the basis a bandwidth of 100 kHz, which is useful for a Manchester-coded 13 kbit/s signal, an effective noise voltage of 115 mV results, said value already being many times higher than the transponder signal voltage, which amounts to about 100 μV.
The noise of the antenna is induced only by the real portion of the impedance and may be estimated as follows:EANT=EANT√{square root over ((4KTRANTΔf))} . . . about 2 nV/√{square root over ((Hz))}wherein K=Boltzmann constant, T=absolute temperature and RANT=real portion of the antenna impedance. With a receiver bandwidth of 100 kHz, an effective noise voltage of 630 nV is achieved. Thus, the power amplifier represents the dominant noise source within the system, and in this manner the signal/noise ratio is determined.
In addition to the undesired noise, there is further interference which makes decoding of the transponder signal more difficult, for example a distortion that results from detuning of the reader antenna 112 and that causes a shift, or offset, in the transfer function within the frequency range. This detuning may be caused by a change in the distance between the antennas, as is depicted in FIG. 3 by the various mutual inductances. As may be seen in FIG. 3, a higher mutual inductance results in that the transfer function is shifted toward higher frequencies. However, since demodulation takes place such that it is synchronous with the generator signal, the movement also appears in the baseband, so that the baseband transfer function is no longer a first-order low-pass function. As a result, the transponder signal is distorted, which makes itself felt by a beat of the transponder signal. If the transponder is implanted near the heart, it will move in the rhythm of the heartbeat, and the mutual inductance between the coils will additionally depend on the mutual orientation of the coils. Thus, the attenuation of the voltage across the reader antenna will also vary, which in turn will make itself felt by a beat in the baseband signal.
The signal/noise ratio is a measure describing the quality of the signal. When assuming a conventional load modulation as is used in the transponder system of FIG. 1, the signal/noise ratio may be calculated as follows:SNR=10 log(Veff/Vnoise)=10 log(141 μV)2/115 mV)2=−58.2 dB.
When using a Manchester-coded signal with 13 kbit/s, a signal/noise ratio of about +10 dB is typically useful for obtaining an acceptable bit error rate (BER). This shows that data transmission is not possible while using conventional load modulation.
In actual fact, conventional load modulation has several disadvantages, which lead to a considerable reduction of the reading range. As was already set forth above, only a very small signal/noise ratio results for data transmission. The sideband signal generated by the load modulation has a spectral distance from the carrier signal that is roughly equal to the data rate. As may be seen from FIG. 4(b), however, the spectral noise power is very high in the vicinity of the carrier signal. The small spectral distance of only a few kH renders filtering impossible. An amplitude variation of the carrier signal, caused by the detuning or by antenna movements, irreversibly superimposes the data signal and renders decoding difficult, if not impossible. In addition, a load modulation wastes energy within the transponder, since during the modulation phase, the modulation resistor is connected to the resonant circuit, and the energy stored within the resonant circuit is transformed into heat during the modulation phase and therefore cannot be reused.
For this reason it is desirable to increase the spectral distance between the reader signal and the data signal; in conventional technology, there are various possibilities of achieving this; however, they do not lead to a useful solution for employing a sensor inside a human body.
A first possible approach is utilization of a so-called sub-carrier as is proposed by the ISO norms 14443 and 10536. A sub-carrier in this context means that the data signal is multiplied, at a constant frequency, by a square-wave signal, for which purpose a frequency of 212 kHz is used, for example. Following the multiplication, both sidebands of the data signal are shifted by a large distance from the carrier signal, specifically in positions where the noise density caused by the power amplifier is lower. However, this technique is disadvantageous since it still employs load modulation, with all of the above-described disadvantages. To eliminate the desired generator signal, the sideband is filtered by a bandpass filter, and the other sideband is not used. Half of the signal energy is thus lost. A further problem is the envelope function of the voltage across the transponder antenna, which limits the possible frequency shift.
FIG. 5 shows the voltage shape over time at a transponder antenna with load modulation while using a sub-carrier. At the time “1”, the modulation resistor is connected. Subsequently, the voltage rapidly decreases to “2”. The energy within the resonant circuit is transformed into heat. The transponder chip is supplied by the load capacity up to the time “3”. At the time “3”, the resistor is released and will now obtain energy for increasing the voltage from the reader. This duration depends on the quality factor of the resonant circuit and continues until the time “4” is reached. From this point onward, the diodes of the rectifier within the transponder chip become conductive, and the rise becomes flatter, since the load capacity of the transponder chip is recharged. The time “5” is the first time for starting the next modulation process. Thereby, the maximum sub-carrier frequency is limited, and simulations have shown that, for this application, a sub-carrier frequency of 70 kHz would be ideal—however with an achievable signal/noise ratio at −20 dB, which is still too low.
A further approach to increasing the spectral distance between the reader signal and the data signal consists in providing magnetically disconnected antennas, FIG. 6 showing a fundamental antenna arrangement for such an implementation. It may be seen in FIG. 6 that the reader antenna 122 comprises a first antenna 122a and a second antenna 122b arranged orthogonally to the first antenna 122a. Likewise, the transponder antenna 110 comprises a first antenna 110a and a second antenna 110b arranged orthogonally to the first antenna 110a. The first antenna 122a of the reader 120 serves to transmit energy to the transponder 100, wherein the energy is received by the first antenna 110a. Within the transponder 100, the antenna 110b arranged orthogonally to the antenna 110 is provided for a data transmission, and within the reader 120, the antenna 122b receives the data. The corresponding field configurations for the energy transmission and the data transmission are also shown in FIG. 6, data transmission being depicted by the continuous line, and energy transmission by the dotted line. Thus, within the reader and within the transponder, the antennas 112b and 110b that may be used for data transmission are arranged orthogonally to the power transmission antennas 122a and 110a, so that data transmission will not be influenced by any undesired coupling of the energy transmission from the reader, including noise. However, this approach has several disadvantages, so that it is not suited for being applied in medical sensor transponders. Firstly, there is not sufficient room for a second antenna within a transponder that is to be implanted by means of a catheter. A second problem is the fact that a useful fixed orientation of the antennas for the implanted transponder is not always guaranteed, and that said implanted transponder also has a very poor energy balance. Additional power may be used for generating a transmission signal using the second antenna. For this purpose, only the power received from the reader by means of the first antenna is available, but losses will occur within the rectifier and the amplifier which drives the second antenna. The efficiency of the rectifier depends on the threshold voltages of the diodes used and on the parasitic capacitances, so that even when using a highly efficient amplifier for driving the second antenna, such as a class C amplifier, 20% of the power will still be lost.
In addition to the above-described methods, there is also the so-called sequential method, which is a kind of time multiplexing. During a first period of time, the power is transmitted from the reader 120 to the transponder 100, where the power is stored within a load capacitor. During a second period of time, the reader 120 switches off the field, and data transmission from the transponder 100 to the reader is performed. The transponder uses the stored energy for generating transmission of the signal, the same antennas being used for transmitting the energy and for transmitting the data. The advantage of this method is that, during data transmission, there will be no disturbing carrier signal from the reader, so that considerably higher signal/noise ratios may be achieved. However, a disadvantage is the requirement of providing a load capacitor having sufficient capacity which more clearly exceeds the dimensions admissible for an implantable transponder. As in the method using orthogonal antennas, losses will also occur within the rectifier and the amplifier, so that power will be lost. In addition, data transmission consumes more time since it has to be interrupted periodically to transmit energy. For continuous pressure data transmission, higher data rates and a buffer within the transponder would thus be useful, so that this method, too, cannot be employed for the above-mentioned applications.
In summary, one may therefore state that the approaches—described in detail above and known from conventional technology—of using passive transponders are not sufficient for supplying a sensor transponder having a limited antenna size with sufficient energy while ensuring safe data transmission, in particular in environments wherein the transponder is surrounded by attenuating material, such as when being used inside a human body.